Vibration detectors and transducers are used in a wide variety of applications. For instance, vibration detectors may be incorporated into buildings, bridges, or other structures to warn of seismic or other potentially destructive vibrations. Similarly, vibration dectectors may provided on industrial equipment such as engines, valves, pumps, fans and the like to indicate dangerous conditions.
One particular class of vibration detectors comprises acoustic detectors, wherein the vibrations to be detected are in the audible range. Such detectors are used in conventional microphones, as well as in more sophisticated speech recognition systems, noise-suppression systems, and the like.
The trend in recent years has been toward replacing conventional vibration detectors and/or microphones with highly miniaturized, micromechanical elements, which among other things, are less expensive to manufacture than their larger predecessors. One type of micromechanical vibration sensor comprises an array of resonators, each having a different resonant frequency, and detection means, such as piezoelectric elements, strain detecting elements, and capacitive elements, for detecting the output generated by the resonance of the resonators. Examples of this type of vibration sensor can be found in U.S. Pat. No. 6,079,274 to Ando et al., U.S. Pat. No. 6,092,422, to Binnig et al., and U.S. Pat. No. 6,223,601 to Harada et al., the contents of each of which are incoporated herein by reference.
One particularly useful application of vibration detectors/transducers has been in the field of hearing enhancement technology. In general, hearing enhancement technology is used to compensate for hearing loss that can not be reversed through medication or surgery. This type of hearing loss, typically caused by malfunctioning of the inner ear or auditory nerve, is known as sensori-neural hearing loss.
Many people with mild-to-moderate sensori-neural hearing loss can use hearing aids to amplify sounds. However, hearing aids are generally ineffective for people with severe or profound sensori-neural hearing loss. Such people often require cochlear implants, which convert sound into electrical impulses that directly stimulate the nerve endings in the cochlea.
Both hearing aids and cochlear implants employ microphones to sense sound and signal processors to analyze and make the sound more recognizable, while filtering noise and ambient sounds. Currently, most signal processors employ digital signal processing (DSP) technology. However this technology requires relatively large and expensive microelectronic chipsets that consume large amounts of power, typically 150-750 mW for a cochlear implant. Consequently, the devices require large battery packs and body-worn accessories to produce the electrical signals needed for the deaf to hear. Furthermore, the battery life is often limited to less than a day, requiring frequent recharging of the devices.
The expense, bulky size, and weight of the current technology means that the majority of the hearing impaired population cannot truly benefit from the technology. Small DSP-based devices are prohibitively expensive to most people. Body-worn accessories and frequent re-charging requirements make the devices less desirable for active people and children, resulting in a significant impact on the quality of life for the hearing impaired members of the population.
Another disadvantage of some currently available hearing devices is that they are not directionally sensitive. In other words, although the devices are capable of analyzing and filtering sounds, their output is not affected by the orientation of the device relative to the incoming sound waves. As a result, the user can hear sounds, but can not tell where the sounds are coming from. This can be a safety issue in certain situations, for instance for drivers or pedestrians who need to know where they are in relation to oncoming vehicles before they can see them.
One suggested alternative to the current technology is to use resonator arrays of the type described in the above-listed patents to Ando at al., Binnig et al, and Harada et al., rather than a single microphone. By adjusting the output efficiency of each resonator, one can accomplish frequency filtering, and remove the need for DSP-based filtering. The resonator array can thus be thought of as a “piano in the ear”, where each key resonates at a specific frequency, making it a mechanical frequency filter.
A simple resonator device based on the above idea was described in Tanaka et al., “A Novel Mechanical Cochlea “Fishbone” with Dual Sensor/Actuator Characteristics,” IEEE/ASME Transactions on Mechtronics, Vol. 3, No. 2, June 1998, pp. 98-105, the contents of which are incorporated in their entirety herein by reference. The resonator device, micromachined from silicon, consists of an array of cantilevers lined along a single transverse base. The “fishbone” (so-called because of its shape similarity to the skeleton of a fish) acts as a multifrequency sensor—each individual cantilever responding to a different acoustic frequency. A prototype of the resonator device displayed good frequency selectivity over a frequency range of 1.4 kHz to 7 kHz, but still had three major drawbacks. First, the natural range of frequencies for this device could not extend to the low frequencies (under 1 kHz). Second, the resonators had a very high efficiency, or Q-factor Q10, causing them to resonate long after the original sound had stopped (this makes an “echo” or “ringing” sound). Third, there was no easy way to convert the vibrating silicon cantilevers into electrical energy.
Accordingly, a need exists for new and improved devices and methods for detecting and processing sounds, and for applying these devices and methods in the field of hearing enhancement technology. More specifically, there is a need for small, low-cost, low-power sound detecting and processing devices that are free of electromagnetic interference and suitable for use in both hearing aids and cochlear implants. Furthermore, such devices should be directionally sensitive, respond to a large range of frequencies, including low frequencies (under 1 kHz), and display suitable damping of the sound after the source has stopped.